Network Working Group                                           K. Naito
Internet-Draft                                              A. Matsumoto
Intended status: Informational                                       NTT
Expires: January 18, 2013                                  July 17, 2012

                          NAT Port Overlapping


   When network address translation (NAT) is used in an address resource
   restricted environment, or when a lot of users are located under a
   NAT device, IP addresses and port resources may be eaten up, and this
   affects user experiences very negatively.  This situation can be
   greatly mitigated by tweaking mapping behavior and session timer
   handling in NAT functions.  This document proposes extension for
   optimizing NAT IP address and port resources in address resource
   restricted environments.  The extension enables simultaneous use of a
   NAT external port for different transport sessions.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 18, 2013.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect

Naito & Matsumoto       Expires January 18, 2013                [Page 1]

Internet-Draft            NAT Port Overlapping                 July 2012

   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

1.  Introduction

   After IPv4 addresses run out, IPv4 address resources will be further
   restricted site-by-site.  If global IPv4 address are shared between
   several clients, assignable port resources at each client will be

   NAT is a tool that is widely used to deal with this IPv4 address
   shortage problem.  However, the demand for resources to provide
   Internet access to users and devices will continue to increase.  IPv6
   is a fundamental solution to this problem, but the deployment of IPv6
   will take time.

   In some cases, e.g. browsing a dynamic web page for a map service, a
   lot of sessions are used by the browser, and a number of ports are
   eaten up in a short time.  What is worse is that when a NAT is
   between a PC and a server, TIME_WAIT state of each TCP connection is
   kept for certain period, typically for four minutes, which consumes
   port resources.  Therefore, new connections cannot be established.

   This problem is caused or worsened by the following behavior.

      In a lot of NAT implementations, a port that is available in NAT
      is allocated for a transport session.That is, a NAT does not use a
      port for multiple sessions simultaneously.

   We propose mechanisms to change the above behavior that make it
   possible to save addresses and ports resources.

Naito & Matsumoto       Expires January 18, 2013                [Page 2]

Internet-Draft            NAT Port Overlapping                 July 2012

2.   NAT resource optimizing extension proposal

2.1.   Apply port overlapping mechanism

   If destination addresses and ports are different at the outgoing
   sessions started by local clients, NAT MAY assign the same external
   port as the source ports at the sessions.  Port overlapping mechanism
   manages mappings between external packets and internal packets by
   looking at and storing the 5-tuple (protocol, source address, source
   port, destination address, destination port) of them.  Thus, enables
   concurrent use of single port for multiple transport sessions, which
   enables NAT to work correctly in IP address resource limited network.


   RFC4787 [RFC4787] and RFC5382 [RFC5382] requires "endpoint-
   independent mapping" at NAT, and port overlapping NAT cannot meet the
   requirement.  This mechanism can degrade the transparency of NAT in
   that its mapping mechanism is endpoint-dependent and makes NAT
   traversal harder.  However, if a NAT adopts endpoint-independent
   mapping together with endpoint-dependent filtering, then the actual
   behavior of the NAT will be the same as port overlapping NAT.  It
   should also be noted that a lot of existing NAT devices(e.g., SEIL,
   FITELnet Series) adopted this port overlapping mechanism.  The
   netfilter, which is a popular packet filtering mechanism for Linux,
   also adopts port overlapping behavior.

3.  Security Considerations

   Security issues are not discussed in this memo.

4.  Normative References

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, September 1981.

   [RFC4787]  Audet, F. and C. Jennings, "Network Address Translation
              (NAT) Behavioral Requirements for Unicast UDP", BCP 127,
              RFC 4787, January 2007.

   [RFC5382]  Guha, S., Biswas, K., Ford, B., Sivakumar, S., and P.
              Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142,
              RFC 5382, October 2008.

Naito & Matsumoto       Expires January 18, 2013                [Page 3]

Internet-Draft            NAT Port Overlapping                 July 2012

Appendix A.  Revision History

   'draft-naito-nat-resource-optimizing-extension-01' was divided into
   two drafts after IETF83 meeting.
   'draft-naito-nat-resource-optimizing-extension-01' containes two
   mechanisms.  One mechanism, port overlapping is written in this
   draft, and the other is written in

Authors' Addresses

   Kengo Naito
   NTT NT Lab
   3-9-11 Midori-Cho
   Musashino-shi, Tokyo  180-8585

   Phone: +81 422 59 4949

   Arifumi Matsumoto
   NTT NT Lab
   3-9-11 Midori-Cho
   Musashino-shi, Tokyo  180-8585

   Phone: +81 422 59 3334

Naito & Matsumoto       Expires January 18, 2013                [Page 4]